TDM links are synchronous circuits, with a constant bit rate governed by the service clock fservice. With a packet network the connection between the ingress and egress frequency is broken, since packets are discontinuous in time. From FIG. 1, the TDM service frequency fservice at the customer premises must be exactly reproduced at the egress of the packet network (fregen). The consequence of a long-term mismatch in frequency is that the queue at the egress of the packet network will either fill up or empty, depending on whether the regenerated clock is slower or faster than the original. This will cause loss of data and degradation of the service.
The relevant standards on circuit emulation services over ATM, ITU standard I.363.1 and ATM Forum standard af-vtoa-0078 refer to the concept of adaptive clock recovery in general terms.